You mean have the re-skinned model be a separate model? Why not just create a directory in the game folder called misc or something, copy the model plus re-skinned textures into it, and then add it to gameinfo.txt like loading any other source game into SFM.

If you mean overriding textures without actually replacing them, you can do that by putting it in usermod with the same filepath. For instance, if you have a replacement for the Sniper's BLU team clothing, put it in usermod/materials/models/player/sniper, and be sure to call it sfm_sniper_blue.vtf. Once you don't need it anymore, simply delete it or move it somewhere else, and the normal texture will be back on the BLU Sniper.

That would be good, but if I'm going to need both at the same time, like in the same video or picture, I want it to be a separate model, my problem is that I downloaded 12 reskins (models that reskin the ones that are already in game), 6 prototype items (proto), 5 millenium items (millenium) and 1 pet Imp (Imp), renamed them and put them into tf_movies and only the pet Imp worked, others were there, but they were invisible, what should I do to get them working withought replacing anything?

I would recommend putting replacement textures in usermod instead of tf_movies, to keep better track of what you added yourself and not mistakenly replace anything important. As far as using multiple skins go, some models have a few skins built into them, so if for instance you need to have the RED Soldier, the BLU one and one custom one, say a green Soldier, you could always override the soldier_blue_blood skin. If however you need more skins than are built into the models, you unfortunately DO need a different model, as adding skins in already made ones is impossible without recompiling it.

And I was just pointing out an easier way to achieve just that without the need for a separate model file, which as I said at the end, you can't do without downloading one or otherwise recompiling an existing model. If it's still what you want to do though, don't forget, Google's you friend.

Like I said: if you want a separate model, you'll need to recompile it with different skin definitions, so that it will point to your new skins. You'll either have to look up how to do that yourself (as I haven't got a clue myself I'm afraid, perhaps someone could help?), or find one somebody else did. I'm however going to once again suggest you'd go the simple route and override unneeded skins on the original models. Believe me, for what you're trying to achieve, it's much less of a hassle.